The present invention relates to a fault detection apparatus and method of detecting faults in an electrical distribution network and more particularly to an apparatus and method for detecting faults in a network utilizing a neutral earthing system.
Three phase distribution networks carry energy along three separate conductors while maintaining a voltage difference between any two of the conductors (line voltages). Under normal operating conditions the three voltages are symmetrical about a neutral point. Voltage measured between a conductor and the neutral point is referred to as the phase voltage.
Where star (Y) connected transformer windings are used at a source station, the star point corresponds to the neutral position. For certain circuits this neutral point is brought out as a fourth conductor and can carry load current for loads connected between a phase and earth. This is the common arrangement for low voltage (LV) circuits and is used extensively in the United States of America.
Each conductor or phase also has a voltage with respect to its surrounding environment, mainly the local ground or earth. Load current normally flows out in one or more of the phases and returns through the other phase wire(s). In the event of a fault, for example a conductor coming in contact with the ground, some of the current flows into the ground. This fault current must find its way back to the source, i.e. into the electrical network. The network neutral point is generally connected to earth at the source to provide such a return path. Fault current for a network earth fault therefore generally flows from the phase conductor, through the earth and back through the neutral-earth connection.
There are many ways to connect the network neutral to earth, for example, it can be solidly connected. In this case large currents can flow in the event of a network earth fault. Such faults can be detected easily but must be cleared rapidly to minimise safety hazards and equipment damage. Solid earthing is predominantly used on cabled networks, where fault rates are lower and faults must be detected and cleared quickly to contain damage to cables.
Alternatively, the neutral point can be left isolated. In this case fault current flows back into the system through a weak capacitive connection between the earth and the remaining phase wires. Relatively little fault current can flow through this capacitive link in the event of a fault and faults are more difficult to detect. However by taking certain precautions, these faults can be tolerated on the network, that is it is not necessary to trip the circuit straight away and interrupt supply to customers. Isolated neutral earthing is predominantly used on rural overhead networks, where faults are much more frequent, damage is less severe and it is desirable to minimise frequent outages.
In another form of neutral earthing, an impedance such as a resistor is used in the neutral-earth link to limit the amount of current that can flow in a fault. Where low impedance devices are used, so that faults can be detected easily but must be cleared quickly, the arrangement is referred to as low impedance earthing. Where high impedance devices are used, fault currents are limited so that damage is limited and faults can be sustained on the network. This arrangement is referred to as a high impedance earthed network. Isolated neutral networks fall into the category of high impedance earthed networks, as the capacitive coupling referred to above is effectively a high impedance link to earth.
High impedance earthed networks offer better operational performance in rural overhead networks when earth faults can be detected reliably. Once a fault is detected the simplest corrective measure is to trip the circuit. However, where frequent faults events occur this can be very disruptive for customers. Where it is desired to maintain supply in the event of a fault, the fault current must be brought to a very low level at the fault site.
There are two method used for reducing the fault current at a fault site. One method is to install an arc-suppression coil in the neutral-earth link at the source station. The coil diverts or tunes out most of the fault current. The second method involves the use of an earthing switch to connect the faulted phase directly to earth in the source station. This switching shorts out the fault diverting the bulk of the fault current directly to the station. This method is referred to as faulted phase earthing (FPE).
The impedance of the fault at the fault site also determines the amount of current that flows in an earth fault. Low impedance faults facilitate current flow allowing the fault to be detected more readily, whereas high impedance faults restrict current flow making detection more difficult. High impedance faults are typically associated with fallen conductors or accidental contact with a live conductor and are extremely dangerous despite the restricted current flow. It is highly desirable to be able to detect such high impedance faults. The ability of a protection system for an electrical distribution network to be able to detect high impedance faults is referred to as its sensitivity.
There are also factors militating against sensitive protection. During the normal operation of a network there is always a certain amount of current flowing to earth through the inherent capacitive links between each phase conductor and earth. When one of the wires is switched, for example, during normal operations or load switching, these currents can be interrupted or become unbalanced. Unbalance currents flow in the earth and in any neutral-earth link, similar to the behaviour of current in an earth fault. As these are normal operational events, the protection system must either discriminate between such events and real faults or it must be de-sensitised to them.
These issues arise in relation to overhead medium voltage (MV) distribution networks which are much more extensive than underground networks. As the conductors are exposed to weathering they are considerably more fault prone. Additionally, bare conductors overground increases the risk of exposing the public to hazards in the event of an accident or plant failure.
In a low impedance earthed network, earth fault protection usually relies on detecting the fault currents associated with a fault and is referred to as over-current protection. A modified over-current protection system utilizes the power flow associated with an earth fault and is referred to as directional over-current protection. Neither protection system can discriminate between high impedance faults and operational events. The sensitivity of these systems is therefore limited by the need to avoid reacting to operational events.
In a high impedance earthed network, the phase voltages during an earth fault are disturbed. These disturbances can be used to detect earth faults in a protection system referred to as voltage displacement protection which is also affected by operational events and must operate with limited sensitivity. To limit spurious operation a threshold of 30% neutral displacement is often used. The sensitivity is thus limited to 30% of the maximum possible fault current, for example, in a 10 kV network with a 15 A maximum fault current, sensitivity would be 5 A. Any fault below 5 A would not be detected.
Wattmetric protection measures residual current on a feeder and forms a product with the neutral voltage displacement from earth. The product gives a directional indication of the source of the fault and can be used to identify faulty feeders. Wattmetric protection techniques however cannot discriminate between earth faults and imbalance effects.
The method of neutral earthing used has a significant impact on the performance of a MV distribution network, particularly when applied to an overhead network. Neutral earthing techniques determine the manner in which earth faults are detected and treated. They also determine the degree of stress, particularly voltage stress, on the network.
In a low impedance neutral system, when an earth fault occurs, the faulted phase must be tripped which has continuity and supply quality implications. With high impedance earthing such earth faults may be sustained which can in turn have safety implications. High impedance earthing imposes higher voltage stress on a network and can lead to cross-country faults which can have much more serious consequences than a simple earth fault. However, high impedance earthing has an overall positive impact on supply continuity and supply quality for overhead networks.
The method of neutral earthing used also determines the fault currents and voltages arising in the network in the event of an earth fault. Earth fault protection systems must therefore be adapted to the neutral earthing technique used. Using low impedance neutral earthing, for example on an existing 20 kV network, conventional over-current or wattmetric type protection may be used to provide a reasonable degree of protection. Where high impedance neutral earthing is required a more sophisticated technique is needed.
If high impedance neutral earthing of a rural MV networks is required, two options become available. Firstly, arc-suppression with a Petersen coil is frequently used for this purpose. However, there are significant difficulties with arc-suppression in rural networks. The widespread use of single phase spurs in a rural network and the consequent phase imbalance cause significant neutral voltages above earth potential and continuous voltage stress on the network. Arc-suppression techniques therefore need to be supplemented with a sophisticated compensation system. Furthermore arc-suppression is not as safe as faulted phase earthing (FPE), where earth faults are sustained on the network.
The second option is to use FPE which provides a simpler and cheaper solution which facilitates handling typically unbalanced rural networks. Lower voltages at fault sites also result in sustainable earth faults with greater safety. FPE utilised alone can cause severe voltage transients on the network when phase to earth switches are opened and closed.
The performance requirements for earth fault detection and handling are very demanding and may be summarised as follows:
High Level of Sensitivity
A very dangerous type of fault is a fallen conductor, which brings high voltage down to ground level within reach of the public. Where the fallen conductor is on the source side of a break, that is, directly connected to the source, there is a reasonable probability of fault detection depending on ground conditions. Where the fallen conductor is on the load side of a break the fault can be considerably more difficult to detect. The fault current must flow through the downstream load and back to the fault. If there is little load connected at that time, for example at night, the load impedance can be quite high.
For example 40 kVA of single phase load at 20 kV would correspond to a load impedance of about 12,000 ohms. This would limit the earth fault current to about 1 amp. The ground potentials at the fault site may be low but somebody dislodging the conductor would be exposed to lethal voltage levels. It is therefore desirable to detect as many of these faults as possible. Detection of such high impedance faults down to a fault current of 1 amp at least is required. This fault current corresponds to a fault impedance of 12,000 ohms, which includes the effect of any limiting load impedance.
Immunity to Imbalances
Where extensive single phase networks are used for example on rural MV networks significant imbalances are common. Furthermore, single pole switching and fuse blowing cause imbalances very similar to earth faults. For example, opening a link or blowing a fuse on a 50 km long single phase spur takes 50 km of conductor from one phase and transfers it onto the other phase through the load. The effect is to cause an unbalanced current of about 1.8 A in the neutral of a of 20 kV network or a neutral displacement of about 10% in the case of a 10 kV isolated neutral network. Similar events in a three phase network have even greater effects especially when the switching occurs close to the source station. Such events are inherent in networks. Sensitive earth fault protection detects such events and therefore it is vital that increasingly sensitive earth fault protection is able to discriminate such events from fault conditions to avoid inappropriate reactions.
Boosters
The use of boosters in an open delta (xcex94) configuration introduces an additional complication. When a network is parallel-connected through a booster, the open delta configuration causes a shift in the position of the neutrals between the two networks. This neutral shift causes a circulating current in the neutrals of an earthed neutral 20 kV network or neutral displacement in the case of a 10 kV network. It is necessary for sensitive earth fault protection to distinguish between such effects and a real earth fault.
Switching
Single pole switching to parallel networks introduces a voltage displacement between the neutral points. If there is a difference in the voltage drops between the networks where they are parallel-connected, the difference is taken up in the neutral points. This difference causes circulating currents in the neutrals, which from the source appear very similar to an earth fault. Again, adequate earth fault protection must be able to discriminate between switching and an earth fault.
Identification of Faulted Feeder
If a faulted phase is to be taken off line it is essential to be able to identify the faulted feeder in the event of an earth fault. In the case of faulted phase earthing (FPE), identification of the faulted phase for switching is required. As there are at least three types of MV overhead network in common rural networks, an integrated solution applicable to all these networks is highly desirable.
The demands on sensitive earth fault protection for rural networks are particularly severe due to the provision of single phase spurs to accommodate the extensive use of single phase construction and operating devices. A high degree of sophistication for a neutral earthed protection system is needed to distinguish between high impedance earth faults and the impact of normal operating events.
There have been several efforts in the United States of America particularly to use the transient effects arising from a fault to detect high impedance faults which has been successful in several aspects. However, different solutions have been developed for differing classes of fault and no universal solution based on analysing such transient effects has evolved. Present systems attempt to amalgamate the existing various techniques into a composite system to apply the most appropriate technique to a particular fault. Alternatively, all the available techniques are applied simultaneously and a consensus derived between them on whether there is a fault or not.
Medium voltage (MV) distribution networks in the United States of America carry the neutral point as a fourth conductor which normally carries load current and the neutral point must be solidly earthed. This arrangement considerably complicates sensitive earth fault protection. In such arrangements there is little option but to pursue transient based detection techniques.
For a balanced three phase network with arc-suppressed neutral earthing, a further form of transient detection technique has been developed in Sweden. This technique is not suitable for rural distribution network, which make extensive use of single phase distribution.
In France, the national utility EDF have developed a technique for analysing the patterns of currents flowing in the faulted feeder and in all neighbouring feeders to sensitively detect earth faults. This system can detect very high impedance faults, identify the feeder in question but it cannot identify the faulted phase. This is necessary to identify the faulted phase for earthing. The EDF system also cannot discriminate sensitively between single phase events and high impedance faults. This problem is not significant as all EDF networks are three phase and do not use single phase switching or protection techniques. This arrangement would not be acceptable for other networks particularly rural networks, where single phase distribution is common.
U.S. Pat. No. 5,659,453 discloses an apparatus for detecting arcing faults on power lines carrying a load current by identifying bursts of each half cycle of the fundamental current. While the apparatus and method shown is highly suited to the detection of arc faults it does not show or suggest a method for detecting permanent, non arcing faults.
It is an object of the present invention to alleviate the disadvantages associated with the prior art and to provide an apparatus and method for detecting faults in an electrical network and discriminating between faults and other network phenomena.
Accordingly there is provided a fault detection apparatus for use in an electrical distribution network of the type comprising:
one or more feeders, each feeder having one or more phase for delivering power to a load connection of the network;
a power source, producing electrical power for the or each feeder; and
a signal acquisition means for communicating with the network measuring network signals;
wherein the signal acquisition means is connected to a comparator and incorporates:
means for identifying and storing a pre-event signal pattern from the electrical distribution network;
monitoring means for detecting an event in the electrical distribution network and activating the signal acquisition means to identify a post-event signal pattern; and
means for delivering the pre-event signal pattern and the post-event signal pattern to the comparator;
the comparator including means for comparatively analysing the pre-event signal pattern and the post-event signal pattern.
Ideally the comparator incorporates means for generating an event characteristic from the pre-event signal pattern and the post-event signal pattern to identify a particular event type.
Preferably the event characteristic includes a faulted feeder discriminator.
Preferably the event characteristic includes a faulted phase discriminator.
In a preferred embodiment the event characteristic is parsable to identify a fault condition on the electrical distribution network.
In one arrangement of the invention the fault condition identified is a source side fault.
In a further arrangement the fault condition identified is a load side fault.
In a particularly preferred embodiment the event characteristic incorporates an operational event register for distinguishing network operations from network faults.
Preferably the operational event register incorporates means for indicating a switching event.
Preferably the operational event register incorporates means for indicating a paralleling event in the electrical distribution network.
In one embodiment the signal acquisition means comprises:
a current transformer for reading a phase current from a phase of a feeder;
a current transformer for reading an earth linkage current; and
a voltage transformer for reading phase voltages.
Preferably the signal acquisition means incorporates signal conditioners.
Preferably the monitoring means incorporates means for assessing instantaneous phase currents and voltages.
Ideally the monitoring means comprises means for analysing time based signals to determine the magnitude and phase of all currents and voltages.
Preferably the comparator incorporates means for comparing instantaneous phase currents and voltages from preceding phase currents and voltages to produces incremental values.
Ideally the apparatus incorporates switching means to prevent damage to the network when a fault condition is detected.
Ideally the apparatus incorporates switching means to prevent injury to personnel when a fault condition is detected.
According to one aspect of the invention there is provided a method for fault detection in an electrical distribution network comprising the steps of:
identifying and storing a pre-event signal pattern from the network;
monitoring the electrical distribution network to detect an event in the network;
identifying and storing a post-event signal pattern in response to the monitored event; and
comparatively analysing the pre-event signal pattern and the post-event signal pattern.
Ideally the step of comparatively analysing the pre-event signal pattern and the post-event signal pattern comprises the steps of:
monitoring instantaneous phase currents and voltages;
comparing instantaneous phase currents and voltages with preceding phase currents and voltages to produce incremental values;
determining whether the incremental values exceed predetermined threshold values or phase angles approach predetermined quantities; and
discriminating currents and voltages generated from values generated during fault switching and normal operating event.
According to another aspect of the invention there is provided an electrical distribution network for discriminating between network operational events and fault conditions using a fault detection apparatus.
Accordingly, the present invention provides a fault detecting apparatus for use in an electrical distribution network, the apparatus comprising:
signal acquisition means;
monitoring means for assessing instantaneous phase currents and voltages;
comparator means for comparing instantaneous phase currents and voltages from preceding phase currents and voltages to produce incremental values;
means for determining whether the incremental values exceed predefined threshold values or phase angles approach predefined quantities;
means for discriminating current and voltage values thus generated from values generated during fault switching and normal operational events; and
switching means to enable precautionary or remedial measures when a fault condition is detected,
whereby a fault detection apparatus of improved sensitivity is realised for mixed three phase and single phase distribution networks.
The signal acquisition means comprises current transformers for reading phase currents on each of the feeders and for reading the earth linkage current and voltage transformers for reading phase voltages. The analogue signals are conditioned, sampled and converted to digital data signals. Signals acquisition includes filtering the digital signals to extract quantities at the distribution network frequency, for example, at 50 Hz.
The monitoring means for assessing phase currents and voltages comprises means for analysing time based signals to determine the magnitude and phase of all currents and voltages. Under certain conditions a trigger is activated to store preceding magnitude and phase information.
The comparator means compares the magnitude and phase information stored to magnitude and phase information subsequently processed.
Where a fault is detected, the type and source of the fault is determined and precautionary or remedial measures are enabled.
For high impedance earthed networks using faulted phased earthing (FPE), the following procedure is used: Initially in a fault condition, a faulted phase earthing (FPE) switch is closed on the faulted phase. A predetermined fault clearing period elapses at which time all magnitude and phase information is stored before the closed FPE switch is opened. The comparator means then compares the stored information with information acquired following FPE switch opening. If the fault is still present, the FPE switch is activated again. Corresponding procedures may be used where other forms of system earthing is implemented.
Where the fault persists or recurs, xe2x80x9clock-outxe2x80x9d is initiated where the FPE switch on the faulted phase is closed, the neutral-earth linkage switch is opened and an alarm is triggered. In this case, manual intervention is required before the FPE switch and linkage switch are returned to their normal operating positions.
In the event of a disturbance condition, where circulating currents are detected, the normally closed neutral earth linkage switch is opened to break the zero sequence current loop and prevent other protection from operating. Following a predetermined clearing period, all magnitude and phase information is stored before reclosing the linkage switch. The comparator means then compares the stored information with information acquired after the neutral-earth linkage switch is closed. If the disturbance is still present, the linkage switch is opened again. If no disturbance is present the monitoring means resets and continues to analyse the time based signal until a disturbance condition is recognised and the trigger is activated.
A manual mode is provided to facilitate the manual activation of FPE switches and the neutral-earth linkage switch. A reset mode is also provided for resetting the fault detecting apparatus after xe2x80x9clock-outxe2x80x9d.
The fault detecting apparatus includes a controller circuit for implementing the signal acquisition means, the monitoring means, the comparator means, the fault determining means and the discriminating means and for processing data from said means, the controller including output drivers for sending actuation signals to the switching means.
The switching means include faulted phase earthing (FPE) switches, a neutral-earth linkage switch and feeder circuit breakers.
The controller circuit operates under a Supervisory Control and Data Acquisition (SCADA) system.
One problem in analysing network signals to detect fault situations is to isolate the currents and voltage displacements arising from the event. The technique of comparing the signals before and after the event is an effective means of isolating those components arising directly from the event for use in further analysis. The resulting signals are used to detect parameter patterns created during the events and the patterns are then associated with a fault or with an operational event to indicate if a fault has occurred. The parameters pattern recognition techniques have been developed from first principles.
The present invention seeks to achieve good discrimination between faults and operational events, it identifies both the faulted feeder and the faulted phase and can provide some information on the nature of a fault or event. The fault detecting apparatus can therefore be used on a high impedance, neutral earthed distribution network, which makes extensive use of single phase circuits and devices. The apparatus may also be used in conjunction with faulted phase earthed (FPE) protection schemes as it can identify the faulted phase for earthing. While the present invention is more applicable to high or low impedance earthed networks it can also be used on solidly earthed networks. The apparatus is unsuitable for use with American 4 wire distribution networks.
The present invention further provides a method of detecting faults in an electrical distribution network, the method comprising:
acquiring data signals;
monitoring instantaneous phase currents and voltages;
comparing instantaneous phase currents and voltages with preceding phase currents and voltages to produce incremental values;
determining whether the incremental values exceed predetermined threshold values or phase angles approach predetermined quantities;
discriminating currents and voltages generated from values generated during fault switching and normal operating events; and
activating precautionary or remedial measures when a fault condition is detected.
On detecting an earth fault, the earth switch for the faulted phase is closed and is re-opened after a short period, such as one second, to check whether the fault has cleared as a transient fault. If the fault persists, the earth switch is closed again and left closed as a permanent fault which calls for manual intervention.
The neutral earthing resistor switch remains closed for the initial short closure of the faulted phase earthing (FPE) switch which short-circuits the neutral resistor for that period. When a permanent fault is indicated and the FPE switch is closed for a longer period, the neutral resistor switch is opened, realising an isolated neutral network. The FPE switch can then be left on indefinitely. Before the FPE switch is again opened, the neutral earth switch is closed to suppress any transient overvoltages which may occur.